The abnormal combustion of an internal combustion engine is conventionally detected by a vibration sensor coupled to the engine, and by analyzing a waveform detected by such sensor. For instance, Japanese Patent Laid-Open No. 2008-524489 (JP '489) discloses how to detect the abnormal combustion, but no concrete configuration of a circuit to acquire the waveform from the vibration sensor is disclosed.
For instance, to securely acquire a waveform from a vibration sensor in a turbo-charged small engine, a waveform of abnormal combustion having 40 fold magnitude of a normal waveform may be accepted. In order to detect the abnormal combustion by using one AD conversion circuit, a big waveform of the abnormal combustion must be attenuated to be within an input range of the AD conversion circuit. Therefore, if the 40 fold magnitude of the normal waveform is attenuated to fit to the input range of the AD conversion circuit, a gain of a gain circuit must be set to have a small value, such as 1/40. In such setting, the abnormal waveform from a small abnormal combustion is attenuated to have a very small waveform, thereby making it very difficult to detect the small abnormal combustion.
In order to resolve such problem, Japanese Patent Laid-Open No. H05-340331 (JP '331), discloses a knocking control apparatus having a large knock determination unit separately from a normal knock determination unit. The apparatus in JP '331 prevents an overflow of a knock detection signal by a large knock, based on a gain switching according to a determination result by the large knock determination unit.
However, the apparatus of JP '331 switches the gain after detecting the size of the waveform. Therefore, the apparatus cannot properly detect a micro knock due to the delay of the gain switching not properly reflecting a violently changing knock detection signal amplitude in each of the knock determination periods.